A storage area network (SAN) may be used to provide centralized data sharing, data backup, and storage management. A storage area network is a high-speed network of shared storage devices. Elements of a SAN include servers, switches, storage controllers, and storage devices. A storage device is any device that principally contains a single disk or multiple disks for storing data for a computer system or computer network. Each server is usually connected to the network by a host bus adapter (HBA) and will include an HBA device driver, which is the software driver for the HBA. The collection of storage devices is sometimes referred to as a storage pool. The storage devices in a SAN can be collocated, which allows for easier maintenance and easier expandability of the storage pool. The network architecture of most SANs is such that all of the storage devices in the storage pool are potentially available to all the servers that are coupled to the SAN. Additional storage devices can be easily added to the storage pool, and these new storage devices will also be accessible from any server on the SAN.
In a computer network that includes a SAN, the server can act as a pathway or transfer agent between the end user and the stored data. Network servers can access a SAN using the Fiber Channel protocol, taking advantage of the ability of a Fibre Channel fabric to serve as a common physical layer for the transport of multiple upper-layer protocols, such as SCSI, IP, and HIPPI, among other examples. With respect to storage, any element of storage on a SAN may be assigned its own SCSI logical unit number (LUN). The LUN address of each storage element is used to identify the storage element within the SAN. Each storage element and its associated LUN are assigned to one or more of the servers of the SAN. Following this assignment, each server will have logical ownership of one or more LUNs, allowing data generated by the server to be stored in the storage devices corresponding to the LUNs owned by the server.
Along with its advantages, the SAN environment also has additional complexities. One of the complexities relates to the relationship between the execution throttle levels of servers and command queue depth settings of storage controllers in a SAN. As noted above, a SAN may be used to provide access to multiple storage devices from multiple host servers. In a SAN, the number of hosts that each storage controller can support is described as the fan out for the storage controller. The fan out value for a storage controller is the number of servers that can access its storage resources. The command queue depth of the storage controller is the maximum number of input/output (I/O) commands that the device can queue up for processing, past which the commands are either dropped or returned with a busy status. Dropping commands or returning a busy status results in a degradation of the performance of the overall system. The maximum number of I/O commands that a server can have outstanding is generally referred to as the server's execution throttle. The execution throttle is typically controlled by configuration settings in the HBA device driver. A SAN will generally operate most efficiently when the command queue of each storage controller of the network is at or near capacity without ever having exceeded capacity.
A difficulty with the selection of execution throttles for the servers of the SAN concerns the possibility that the execution throttle of a server will be set too high or too low. When an execution throttle value is set too high, that is, when the sum of execution throttles on host servers that own LUNs on a particular storage device exceeds the command queue depth for that storage device, the total I/O demand from the servers is too high as compared to the command capacity, as measured by the command queue depth, of the storage controllers of the SAN, resulting in dropped commands or busy signals returned by the storage controllers. In addition, when an execution throttle value is set too low on a particular server, such as when a low setting is desired in order to insure that the I/O demand from the servers does not exceed the command capacity of the coupled storage controllers, the performance of the server may be severely affected by the inability to process commands fast enough to satisfy the demands of the operating system and application programs that generate the commands.
For many SANs and SAN administrators, a common method for selecting the execution throttle levels of the servers of the SAN is to set the execution throttle for each host to a value that is determined at setup by dividing the smallest command queue depth value of all of the storage controllers on the SAN by the number of servers. This method, however, does not take into account the possibility of varying command queue depths among the several storage controllers on the SAN. As a result, all storage controllers, including those having the capacity for higher command queue depths, are treated as though they have the capacity of the storage controller with the smallest queue depth. Thus, one difficulty of setting a server execution throttle according to this technique is the likelihood that many storage controllers on the SAN will operate at less than their maximum capacity.
Another difficulty of establishing the server execution throttles of the servers of the SAN concerns the process of individually or manually adjusting the execution throttles of one or more of the servers on the SAN. This task is often logistically challenging, burdensome, and time-consuming. An administrator of the SAN must calculate a new value for the execution throttle and must manually reset the execution throttle of every server on the SAN. The problem of resetting server execution throttle is exacerbated when a SAN has multiple storage devices that have different queue depths. Therefore, a method is needed to calculate and reset the execution throttle of servers on the SAN in a manner that is less burdensome and time-consuming to the administrator of the SAN. Another issue with the current method of managing the execution throttle of the servers on the SAN relates to the manual process that the administrator must perform to calculate and set the execution throttle of all the servers on the SAN. Because the process is manual in nature, relying primarily on hand calculations and the manual setting of the execution throttle of each server on the SAN, the process is subject to errors that are potentially very difficult to troubleshoot.